1. Field of the Invention
The present invention related to a semiconductor memory device.
2. Related Art
Regardless of whether a memory is a volatile memory or a nonvolatile memory, disturbance occurs in data of unselected memory cells at a data reading time or at a data writing time. For example, at the time of detecting data of a selected memory cell connected to a selected word line, a potential of a bit line connected to a sense amplifier is also applied to unselected memory cells connected to unselected word lines. In this case, data stored in the unselected memory cells is disturbed by the potential of the bit line. The disturbance is a phenomenon of degrading data stored in the unselected memory cells at the time of accessing the selected memory cell. Disturbance is one of causes of decreasing a signal difference between data “0” and data “1”. In a volatile memory, a signal difference between data “0” and data “1” is gradually degraded even during a data holding time having no access.
To solve this degradation of data due to a disturbance characteristic and a data holding characteristic, a refresh operation is performed. The refresh operation is an operation of recovering data from data degraded at a data holding time or recovering data from data degraded by disturbance.
However, access by data reading or data writing is performed irregularly. Therefore, the refresh operation to solve the problem of disturbance needs to be performed frequently irrespective of actual access frequency by taking the worst case into consideration. This causes increase of power consumption.